Anvil bridge stent design

ABSTRACT

A stent or other intraluminal medical device may be constructed utilizing a series of anvil bridges interposed between a series of fixed bridges to reduce the likelihood of deformation during stent loading and stent deployment without sacrificing overall stent flexibility. The stent comprises a plurality of hoops interconnected by a plurality of fixed bridges. The stent also comprises a plurality of anvil bridges, which transmit forces only when the stent is subjected to compressive axial loading. The stent may also comprise markers formed from housings integral with the stent and marker inserts having a higher radiopacity than the stent. This design provides for more precise placement and post-procedural visualization in a vessel, by increasing the radiopacity of the stent under X-ray fluoroscopy. The housings are formed integral to the stent and the marker inserts are made from a material close in the galvanic series to the stent material and sized to substantially minimize the effect of galvanic corrosion. The housings are also shaped to minimize their impact on the overall profile of the stent.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to stents having a modified bridgedesign and more particularly to stents having an anvil bridge design. Inaddition the present invention relates to intraluminal devices, and moreparticularly to intraluminal devices, such as stents, incorporatingintegral markers for increasing the radiopacity thereof.

[0003] 2. Discussion of Related Art

[0004] Percutaneous transluminal angioplasty (PTA) is a therapeuticmedical procedure used to increase blood flow through an artery. In thisprocedure, the angioplasty balloon is inflated within the stenosedvessel, or body passageway, in order to shear and disrupt the wallcomponents of the vessel to obtain an enlarged lumen. With respect toarterial stenosed lesions, the relatively incompressible plaque remainsunaltered, while the more elastic medial and adventitial layers of thebody passageway stretch around the plaque. This process producesdissection, or a splitting and tearing, of the body passageway walllayers, wherein the intima, or internal surface of the artery or bodypassageway, suffers fissuring. This dissection forms a “flap” ofunderlying tissue, which may reduce the blood flow through the lumen, orcompletely block the lumen. Typically, the distending intraluminalpressure within the body passageway can hold the disrupted layer, orflap, in place. If the intimal flap created by the balloon dilationprocedure is not maintained in place against the expanded intima, theintimal flap can fold down into the lumen and close off the lumen, ormay even become detached and enter the body passageway. When the intimalflap closes off the body passageway, immediate surgery is necessary tocorrect the problem.

[0005] Recently, transluminal prostheses have been widely used in themedical arts for implantation in blood vessels, biliary ducts, or othersimilar organs of the living body. These prostheses are commonlyreferred to as stents and are used to maintain, open, or dilate tubularstructures. An example of a commonly used stent is given in U.S. Pat.No. 4,733,665 to Palmaz. Such stents are often referred to as balloonexpandable stents. Typically the stent is made from a solid tube ofstainless steel. Thereafter, a series of cuts are made in the wall ofthe stent. The stent has a first smaller diameter, which permits thestent to be delivered through the human vasculature by being crimpedonto a balloon catheter. The stent also has a second, expanded diameter,upon application of a radially, outwardly directed force, by the ballooncatheter, from the interior of the tubular shaped member.

[0006] However, one concern with such stents is that they are oftenimpractical for use in some vessels such as the carotid artery. Thecarotid artery is easily accessible from the exterior of the human body,and is close to the surface of the skin. A patient having a balloonexpandable stent made from stainless steel or the like, placed in theircarotid artery, might be susceptible to severe injury through day-to-dayactivity. A sufficient force placed on the patient's neck could causethe stent to collapse, resulting in injury to the patient. In order toprevent this, self-expanding stents have been proposed for use in suchvessels. Self-expanding stents act like springs and will recover totheir expanded or implanted configuration after being crushed.

[0007] One type of self-expanding stent is disclosed in U.S. Pat. No.4,655,771. The stent disclosed in U.S. Pat. No. 4,655,771 has a radiallyand axially flexible, elastic tubular body with a predetermined diameterthat is variable under axial movement of the ends of the body relativeto each other and which is composed of a plurality of individually rigidbut flexible and elastic thread elements defining a radiallyself-expanding helix. This type of stent is known in the art as a“braided stent” and is so designated herein. Placement of such stents ina body vessel can be achieved by a device which comprises an outercatheter for holding the stent at its distal end, and an inner pistonwhich pushes the stent forward once it is in position.

[0008] However, braided stents have many disadvantages. They typicallydo not have the necessary radial strength to effectively hold open adiseased vessel. In addition, the plurality of wires or fibers used tomake such stents could become dangerous if separated from the body ofthe stent, where they could pierce through the vessel. Therefore, therehas been a desire to have a self-expanding stent which is cut from atube of metal, which is the common manufacturing method for manycommercially available balloon-expandable stents. In order tomanufacture a self-expanding stent cut from a tube, the alloy used wouldpreferably exhibit superelastic or psuedoelastic characteristics at bodytemperature, so that it is crush recoverable.

[0009] The prior art makes reference to the use of alloys such asNitinol (Ni—Ti alloy), which have shape memory and/or superelasticcharacteristics, in medical devices which are designed to be insertedinto a patient's body. The shape memory characteristics allow thedevices to be deformed to facilitate their insertion into a body lumenor cavity and then be heated within the body so that the device returnsto its original shape. Superelastic characteristics, on the other hand,generally allow the metal to be deformed and restrained in the deformedcondition to facilitate the insertion of the medical device containingthe metal into a patient's body, with such deformation causing the phasetransformation. Once within the body lumen, the restraint on thesuperelastic member can be removed, thereby reducing the stress thereinso that the superelastic member can return to its original un-deformedshape by the transformation back to the original phase.

[0010] Alloys having shape memory/superelastic characteristics generallyhave at least two phases. These phases are a martensite phase, which hasa relatively low tensile strength and which is stable at relatively lowtemperatures, and an austenite phase, which has a relatively hightensile strength and which is stable at temperatures higher than themartensite phase.

[0011] Shape memory characteristics are imparted to the alloy by heatingthe metal at a temperature above which the transformation from themartensite phase to the austenite phase is complete, i.e. a temperatureabove which the austenite phase is stable (the Af temperature). Theshape of the metal during this heat treatment is the shape “remembered.”The heat-treated metal is cooled to a temperature at which themartensite phase is stable, causing the austenite phase to transform tothe martensite phase. The metal in the martensite phase is thenplastically deformed, e.g. to facilitate the entry thereof into apatient's body. Subsequent heating of the deformed martensite phase to atemperature above the martensite to austenite transformation temperaturecauses the deformed martensite phase to transform to the austenitephase, and during this phase transformation the metal reverts back toits original shape if unrestrained. If restrained, the metal will remainmartensitic until the restraint is removed.

[0012] Methods of using the shape memory characteristics of these alloysin medical devices intended to be placed within a patient's body presentoperational difficulties. For example, with shape memory alloys having astable martensite temperature below body temperature, it is frequentlydifficult to maintain the temperature of the medical device containingsuch an alloy sufficiently below body temperature to prevent thetransformation of the martensite phase to the austenite phase when thedevice was being inserted into a patient's body. With intravasculardevices formed of shape memory alloys having martensite-to-austenitetransformation temperatures well above body temperature, the devices canbe introduced into a patient's body with little or no problem, but theymust be heated to the martensite-to-austenite transformation temperaturewhich is frequently high enough to cause tissue damage.

[0013] When stress is applied to a specimen of a metal such as Nitinolexhibiting superelastic characteristics at a temperature above which theaustenite is stable (i.e. the temperature at which the transformation ofmartensite phase to the austenite phase is complete), the specimendeforms elastically until it reaches a particular stress level where thealloy then undergoes a stress-induced phase transformation from theaustenite phase to the martensite phase. As the phase transformationproceeds, the alloy undergoes significant increases in strain but withlittle or no corresponding increases in stress. The strain increaseswhile the stress remains essentially constant until the transformationof the austenite phase to the martensite phase is complete. Thereafter,further increases in stress are necessary to cause further deformation.The martensitic metal first deforms elastically upon the application ofadditional stress and then plastically with permanent residualdeformation.

[0014] If the load on the specimen is removed before any permanentdeformation has occurred, the martensitic specimen will elasticallyrecover and transform back to the austenite phase. The reduction instress first causes a decrease in strain. As stress reduction reachesthe level at which the martensite phase transforms back into theaustenite phase, the stress level in the specimen will remainessentially constant (but substantially less than the constant stresslevel at which the austenite transforms to the martensite) until thetransformation back to the austenite phase is complete, i.e. there issignificant recovery in strain with only negligible corresponding stressreduction. After the transformation back to austenite is complete,further stress reduction results in elastic strain reduction. Thisability to incur significant strain at relatively constant stress uponthe application of a load, and to recover from the deformation upon theremoval of the load, is commonly referred to as superelasticity orpseudoelasticity. It is this property of the material which makes ituseful in manufacturing tube cut self-expanding stents.

[0015] A concern associated with self-expanding stents is that of thecompressive forces associated with stent loading and stent deployment.In stent designs having periodically positioned bridges, the resultinggaps between unconnected loops may be disadvantageous, especially duringloading into a stent delivery system and subsequent deployment from astent delivery system. In both the loading and deployment situations,the stent is constrained to a small diameter and subjected to highcompressive axial forces. These forces are transmitted axially throughthe stent by the connecting bridges and may cause undesirable bucklingor compression of the adjacent hoops in the areas where the loops arenot connected by bridges.

[0016] One additional concern with stents and with other medical devicesformed from superelastic materials, is that they may exhibit reducedradiopacity under X-ray fluoroscopy. To overcome this problem, it iscommon practice to attach markers, made from highly radiopaquematerials, to the stent, or to use radiopaque materials in plating orcoating processes. Those materials typically include gold, platinum, ortantalum. The prior art makes reference to these markers or processes inU.S. Pat. No. 5,632,771 to Boatman et al., U.S. Pat. No. 6,022,374 toImran, U.S. Pat. No. 5,741,327 to Frantzen, U.S. Pat. No. 5,725,572 toLam et al., and U.S. Pat. No. 5,800,526 to Anderson et al. However, dueto the size of the markers and the relative position of the materialsforming the markers in the galvanic series versus the position of thebase metal of the stent in the galvanic series, there is a certainchallenge to overcome; namely, that of galvanic corrosion. Also, thesize of the markers increases the overall profile of the stent. Inaddition, typical markers are not integral to the stent and thus mayinterfere with the overall performance of the stent as well as becomedislodged from the stent. Also, typical markers are used to indicaterelative position within the lumen and not whether the device is in thedeployed or undepolyed position.,

SUMMARY OF THE INVENTION

[0017] The present invention overcomes the disadvantages associated withundesirable loading effects during stent loading and stent deployment asbriefly discussed above. The present invention also overcomes many ofthe disadvantages associated with reduced radiopacity exhibited byself-expanding stents, balloon-expandable stents, and other medicaldevices as briefly discussed above.

[0018] In accordance with one aspect, the present invention is directedto an intraluminal medical device. The intraluminal medical devicecomprises a plurality of hoops forming a substantially tubular memberhaving front and back open ends, one or more bridges interconnecting theplurality of hoops at predetermined positions to form the substantiallytubular member, and at least one anvil bridge positioned between each ofthe plurality of hoops, the at least one anvil bridge including a firstsection which makes abutting contact with a section of at least one loopon an adjacent hoop when the intraluminal medical device is constrainedand under compressive axial loading. Each of the plurality of hoopscomprises a plurality of struts and a plurality of loops connectingadjacent struts.

[0019] Stent structures are often constructed of radially expandingmembers or hoops connected by bridge elements. In certain stent designs,the bridge elements may connect every tip or loop of the radiallyexpanding members or hoops to a corresponding tip or loop of an adjacentradially expanding member or hoop. This type of design provides for aless flexible stent. In other stent designs, the bridge elements do notconnect every set of tips or loops, but rather, the bridges are placedperiodically. When bridges are periodically spaced, open gaps may existbetween unconnected tips or loops. This design affords increasedflexibility, however, potential deformation of the unconnected tips orloops may occur when the stent is subject to compressive axial loading,for example, during loading of the stent into the stent delivery systemor during deployment of the stent. The anvil bridge design of thepresent invention may be utilized to effectively fill the gap betweenadjacent unconnected tips or loops without serving as a structuralconnection point between such tips or loops. Accordingly, there is nosacrifice in terms of flexibility.

[0020] In addition, the anvil bridge design serves to increase thesurface area of the stent. This increased surface area may be utilizedto modify a drug release profile by increasing the amount of drugavailable for drug delivery. Essentially, increased surface area on thestent allows for more drug coating thereon.

[0021] The intraluminal medical device of the present invention mayutilize high radiopacity markers to ensure proper positioning of thedevice within a lumen. The markers comprise a housing which is integralto the device itself, thereby ensuring minimal interference withdeployment and operation of the device. The housings are also shaped tominimally impact the overall profile of the stent. For example, aproperly shaped housing allows a stent to maintain a radiopaque stentmarker size utilized in a seven French delivery system to fit into a sixFrench delivery system. The markers also comprise a properly sizedmarker insert having a higher radiopacity than the material forming thedevice itself. The marker insert is sized to match the curvature of thehousing thereby ensuring a tight and unobtrusive fit. The marker insertsare made from a material close in the galvanic series to the devicematerial and sized to substantially minimize the effect of galvaniccorrosion.

[0022] The improved radiopacity intraluminal medical device of thepresent invention provides for more precise placement andpost-procedural visualization in a lumen by increasing the radiopacityof the device under X-ray fluoroscopy. Given that the marker housingsare integral to the device, they are simpler and less expensive tomanufacture than markers that have to be attached in a separate process.

[0023] The improved radiopacity intraluminal medical device of thepresent invention is manufactured utilizing a process which ensures thatthe marker insert is securely positioned within the marker housing. Themarker housing is laser cut from the same tube and is integral to thedevice. As a result of the laser cutting process, the hole in the markerhousing is conical in the radial direction with the outer surfacediameter being larger than the inner surface diameter. The conicaltapering effect in the marker housing is beneficial in providing aninterference fit between the marker insert and the marker housing toprevent the marker insert from being dislodged once the device isdeployed. The marker inserts are loaded into a crimped device bypunching a disk from annealed ribbon stock and shaping it to have thesame radius of curvature as the marker housing. Once the disk is loadedinto the marker housing, a coining process is used to properly seat themarker below the surface of the housing. The coining punch is alsoshaped to maintain the same radius of curvature as the marker housing.The coining process deforms the marker housing material to form aprotrusion, thereby locking in the insert or disk.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The foregoing and other aspects of the present invention willbest be appreciated with reference to the detailed description of theinvention in conjunction with the accompanying drawings, wherein:

[0025]FIG. 1 is a perspective view of an exemplary stent in itscompressed state which may be utilized in conjunction with the presentinvention.

[0026]FIG. 2 is a sectional, flat view of the stent shown in FIG. 1.

[0027]FIG. 3 is a perspective view of the stent shown in FIG. 1 butshowing it in its expanded state.

[0028]FIG. 4 is an enlarged sectional view of the stent shown in FIG. 3.

[0029]FIG. 5 is an enlarged view of section of the stent shown in FIG.2.

[0030]FIG. 6 is a view similar to that of FIG. 2 but showing analternate embodiment of the stent.

[0031]FIG. 7 is a perspective view of the stent of FIG. 1 having aplurality of markers attached to the ends thereof in accordance with thepresent invention.

[0032]FIG. 8 is a cross-sectional view of a marker in accordance withthe present invention.

[0033]FIG. 9 is an enlarged perspective view of an end of the stent withthe markers forming a substantially straight line in accordance with thepresent invention.

[0034]FIG. 10 is a simplified partial cross-sectional view of a stentdelivery apparatus having a stent loaded therein, which can be used witha stent made in accordance with the present invention.

[0035]FIG. 11 is a view similar to that of FIG. 10 but showing anenlarged view of the distal end of the apparatus.

[0036]FIG. 12 is a perspective view of an end of the stent with themarkers in a partially expanded form as it emerges from the deliveryapparatus in accordance with the present invention.

[0037]FIG. 13 is an enlarged perspective view of an end of the stentwith modified markers in accordance with an alternate exemplaryembodiment of the present invention.

[0038]FIG. 14 is an enlarged perspective view of an end of the stentwith modified markers in accordance with another alternate exemplaryembodiment of the present invention.

[0039]FIG. 15 is a sectional, flat view of an exemplary embodiment of asplit-bridge stent in accordance with the present invention.

[0040]FIG. 16 is a perspective view of the stent illustrated in FIG. 15,but showing the stent in the expanded state.

[0041]FIG. 17 is a sectional, flat view of an exemplary embodiment of ananvil bridge stent in accordance with the present invention.

[0042]FIG. 18 is a perspective view of the stent illustrated in FIG. 17,but showing the stent in the expanded state.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] While the present invention may be used on or in connection withany number of medical devices, including stents, for ease ofexplanation, one exemplary embodiment of the invention with respect toself-expanding Nitinol stents will be described in detail. There isillustrated in FIGS. 1 and 2, a stent 100, which may be utilized inconnection with the present invention. FIGS. 1 and 2 illustrate theexemplary stent 100 in its unexpanded or compressed state. The stent 100is preferably made from a superelastic alloy such as Nitinol. Mostpreferably, the stent 100 is made from an alloy comprising from about50.0 percent (as used herein these percentages refer to weightpercentages) Ni to about 60 percent Ni, and more preferably about 55.8percent Ni, with the remainder of the alloy being Ti. Preferably, thestent 100 is designed such that it is superelastic at body temperature,and preferably has an Af in the range from about twenty-four degrees C.to about thirty-seven degrees C. The superelastic design of the stent100 makes it crush recoverable, which, as discussed above, makes ituseful as a stent or frame for any number of vascular devices indifferent applications.

[0044] Stent 100 is a tubular member having front and back open ends 102and 104 and a longitudinal axis 106 extending therebetween. The tubularmember has a first smaller diameter, FIGS. 1 and 2, for insertion into apatient and navigation through the vessels, and a second largerdiameter, FIGS. 3 and 4, for deployment into the target area of avessel. The tubular member is made from a plurality of adjacent hoops108, FIG. 1 showing hoops 108(a)-108(d), extending between the front andback ends 102 and 104. The hoops 108 include a plurality of longitudinalstruts 110 and a plurality of loops 112 connecting adjacent struts,wherein adjacent struts are connected at opposite ends so as to form asubstantially S or Z shape pattern. The loops 112 are curved,substantially semi-circular with symmetrical sections about theircenters 114.

[0045] Stent 100 further includes a plurality of bridges 116 whichconnect adjacent hoops 108 and which can best be described in detail byreferring to FIG. 5. Each bridge 116 has two ends 118 and 120. Thebridges 116 have one end attached to one strut and/or loop, and anotherend attached to a strut and/or loop on an adjacent hoop. The bridges 116connect adjacent struts together at bridge to loop connection points 122and 124. For example, bridge end 118 is connected to loop 114(a) atbridge to loop connection point 122, and bridge end 120 is connected toloop 114(b) at bridge to loop connection point 124. Each bridge to loopconnection point has a center 126. The bridge to loop connection pointsare separated angularly with respect to the longitudinal axis. That is,the connection points are not immediately opposite each other.Essentially, one could not draw a straight line between the connectionpoints, wherein such line would be parallel to the longitudinal axis ofthe stent.

[0046] The above-described geometry helps to better distribute strainthroughout the stent, prevents metal-to-metal contact when the stent isbent, and minimizes the opening size between the struts, loops andbridges. The number of and nature of the design of the struts, loops andbridges are important factors when determining the working propertiesand fatigue life properties of the stent. It was previously thought thatin order to improve the rigidity of the stent, that struts should belarge, and therefore there should be fewer struts per hoop. However, ithas now been discovered that stents having smaller struts and morestruts per hoop actually improve the construction of the stent andprovide greater rigidity. Preferably, each hoop has between twenty-fourto thirty-six or more struts. It has been determined that a stent havinga ratio of number of struts per hoop to strut length L (in inches) whichis greater than four hundred has increased rigidity over prior artstents, which typically have a ratio of under two hundred. The length ofa strut is measured in its compressed state parallel to the longitudinalaxis 106 of the stent 100 as illustrated in FIG. 1.

[0047] As seen from a comparison of FIGS. 2 and 3, the geometry of thestent 100 changes quite significantly as the stent 100 is deployed fromits unexpanded state to its expanded state. As a stent undergoesdiametric change, the strut angle and strain levels in the loops andbridges are affected. Preferably, all of the stent features will strainin a predictable manner so that the stent is reliable and uniform instrength. In addition, it is preferable to minimize the maximum strainexperienced by struts loops and bridges, since Nitinol properties aremore generally limited by strain rather than by stress. As will bediscussed in greater detail below, the stent sits in the delivery systemin its un-expanded state as shown in FIGS. 10 and 11. As the stent isdeployed, it is allowed to expand towards its expanded state, as shownin FIG. 3, which preferably has a diameter which is the same or largerthan the diameter of the target vessel. Nitinol stents made from wiredeploy in much the same manner, and are dependent upon the same designconstraints, as laser cut stents. Stainless steel stents deploysimilarly in terms of geometric changes as they are assisted by forcesfrom balloons or other devices.

[0048] In trying to minimize the maximum strain experienced by featuresof the stent, the present invention utilizes structural geometries whichdistribute strain to areas of the stent which are less susceptible tofailure than others. For example, one of the most vulnerable areas ofthe stent is the inside radius of the connecting loops. The connectingloops undergo the most deformation of all the stent features. The insideradius of the loop would normally be the area with the highest level ofstrain on the stent. This area is also critical in that it is usuallythe smallest radius on the stent. Stress concentrations are generallycontrolled or minimized by maintaining the largest radii possible.Similarly, we want to minimize local strain concentrations on the bridgeand bridge connection points. One way to accomplish this is to utilizethe largest possible radii while maintaining feature widths which areconsistent with applied forces. Another consideration is to minimize themaximum open area of the stent. Efficient utilization of the originaltube from which the stent is cut increases stent strength and itsability to trap embolic material.

[0049] Many of these design objectives have been accomplished by anexemplary embodiment of the present invention, illustrated in FIGS. 1, 2and 5. As seen from these figures, the most compact designs whichmaintain the largest radii at the loop to bridge connections arenon-symmetric with respect to the centerline of the strut connectingloop. That is, loop to bridge connection point centers 126 are offsetfrom the center 114 of the loops 112 to which they are attached. Thisfeature is particularly advantageous for stents having large expansionratios, which in turn requires them to have extreme bending requirementswhere large elastic strains are required. Nitinol can withstandextremely large amounts of elastic strain deformation, so the abovefeatures are well suited to stents made from this alloy. This featureallows for maximum utilization of Ni—Ti or other material properties toenhance radial strength, to improve stent strength uniformity, toimprove fatigue life by minimizing local strain levels, to allow forsmaller open areas which enhance entrapment of embolic material, and toimprove stent apposition in irregular vessel wall shapes and curves.

[0050] As seen in FIG. 5, stent 100 comprises strut connecting loops 112having a width W1, as measured at the center 114 parallel to axis 106,which are greater than the strut widths W2, as measured perpendicular toaxis 106 itself. In fact, it is preferable that the thickness of theloops vary so that they are thickest near their centers. This increasesstrain deformation at the strut and reduces the maximum strain levels atthe extreme radii of the loop. This reduces the risk of stent failureand allows one to maximize radial strength properties. This feature isparticularly advantageous for stents having large expansion ratios,which in turn requires them to have extreme bending requirements wherelarge elastic strains are required. Nitinol can withstand extremelylarge amounts of elastic strain deformation, so the above features arewell suited to stents made from this alloy. As stated above, thisfeature allows for maximum utilization of Ni—Ti or other materialproperties to enhance radial strength, to improve stent strengthuniformity, to improve fatigue life by minimizing local strain levels,to allow for smaller open areas which enhance entrapment of embolicmaterial, and to improve stent apposition in irregular vessel wallshapes and curves.

[0051] As mentioned above, bridge geometry changes as a stent isdeployed from its compressed state to its expanded state and vise-versa.As a stent undergoes diametric change, strut angle and loop strain isaffected. Since the bridges are connected to either the loops, struts orboth, they are affected. Twisting of one end of the stent with respectto the other, while loaded in the stent delivery system, should beavoided. Local torque delivered to the bridge ends displaces the bridgegeometry. If the bridge design is duplicated around the stent perimeter,this displacement causes rotational shifting of the two loops beingconnected by the bridges. If the bridge design is duplicated throughoutthe stent, as in the present invention, this shift will occur down thelength of the stent. This is a cumulative effect as one considersrotation of one end with respect to the other upon deployment. A stentdelivery system, such as the one described below, will deploy the distalend first, then allow the proximal end to expand. It would beundesirable to allow the distal end to anchor into the vessel wall whileholding the stent fixed in rotation, then release the proximal end. Thiscould cause the stent to twist or whip in rotation to equilibrium afterit is at least partially deployed within the vessel. Such whippingaction may cause damage to the vessel.

[0052] However, one exemplary embodiment of the present invention, asillustrated in FIGS. 1 and 2, reduces the chance of such eventshappening when deploying the stent. By mirroring the bridge geometrylongitudinally down the stent, the rotational shift of the Z-sections orS-sections may be made to alternate and will minimize large rotationalchanges between any two points on a given stent during deployment orconstraint. That is, the bridges 116 connecting loop 108(b) to loop108(c) are angled upwardly from left to right, while the bridgesconnecting loop 108(c) to loop 108(d) are angled downwardly from left toright. This alternating pattern is repeated down the length of the stent100. This alternating pattern of bridge slopes improves the torsionalcharacteristics of the stent so as to minimize any twisting or rotationof the stent with respect to any two hoops. This alternating bridgeslope is particularly beneficial if the stent starts to twist in vivo.As the stent twists, the diameter of the stent will change. Alternatingbridge slopes tend to minimize this effect. The diameter of a stenthaving bridges which are all sloped in the same direction will tend togrow if twisted in one direction and shrink if twisted in the otherdirection. With alternating bridge slopes this effect is minimized andlocalized.

[0053] Preferably, stents are laser cut from small diameter tubing. Forprior art stents, this manufacturing process led to designs withgeometric features, such as struts, loops and bridges, having axialwidths W2, W1 and W3, respectively, which are larger than the tube wallthickness T (illustrated in FIG. 3). When the stent is compressed, mostof the bending occurs in the plane that is created if one were to cutlongitudinally down the stent and flatten it out. However, for theindividual bridges, loops and struts, which have widths greater thantheir thickness, there is a greater resistance to this in-plane bendingthan to out-of-plane bending. Because of this, the bridges and strutstend to twist, so that the stent as a whole may bend more easily. Thistwisting is a buckling condition which is unpredictable and can causepotentially high strain.

[0054] However, this problem has been solved in an exemplary embodimentof the present invention, as illustrated in FIGS. 1-5. As seen fromthese figures, the widths of the struts, hoops and bridges are equal toor less than the wall thickness of the tube. Therefore, substantiallyall bending and, therefore, all strains are “out-of-plane.” Thisminimizes twisting of the stent which minimizes or eliminates bucklingand unpredictable strain conditions. This feature is particularlyadvantageous for stents having large expansion ratios, which in turnrequires them to have extreme bending requirements where large elasticstrains are required. Nitinol, as stated above, can withstand extremelylarge amounts of elastic strain deformation, so the above features arewell suited to stents made from this alloy. This feature allows formaximum utilization of Ni—Ti or other material properties to enhanceradial strength, to improve stent strength uniformity, to improvefatigue life by minimizing local strain levels, to allow for smalleropen areas which enhance entrapment of embolic material, and to improvestent apposition in irregular vessel wall shapes and curves.

[0055] An alternate exemplary embodiment of a stent that may be utilizedin conjunction with the present invention is illustrated in FIG. 6. FIG.6 shows stent 200 which is similar to stent 100 illustrated in FIGS.1-5. Stent 200 is made from a plurality of adjacent hoops 202, FIG. 6showing hoops 202(a)-202(d). The hoops 202 include a plurality oflongitudinal struts 204 and a plurality of loops 206 connecting adjacentstruts, wherein adjacent struts are connected at opposite ends so as toform a substantially S or Z shape pattern. Stent 200 further includes aplurality of bridges 208 which connect adjacent hoops 202. As seen fromthe figure, bridges 208 are non-linear and curve between adjacent hoops.Having curved bridges allows the bridges to curve around the loops andstruts so that the hoops can be placed closer together which in turn,minimizes the maximum open area of the stent and increases its radialstrength as well. This can best be explained by referring to FIG. 4. Theabove described stent geometry attempts to minimize the largest circlewhich could be inscribed between the bridges, loops and struts, when thestent is expanded. Minimizing the size of this theoretical circlegreatly improves the stent because it is then better suited to provideconsistent scaffolding support to support the vessel and trap embolicmaterial once it is inserted into the patient.

[0056] As mentioned above, it is preferred that the stent of the presentinvention be made from a superelastic alloy and most preferably made ofan alloy material having greater than 50.5 atomic percentage Nickel andthe balance Titanium. Greater than 50.5 atomic percentage Nickel allowsfor an alloy in which the temperature at which the martensite phasetransforms completely to the austenite phase (the Af temperature) isbelow human body temperature, and preferably is about twenty-fourdegrees C. to about thirty-seven degrees C., so that austenite is theonly stable phase at body temperature.

[0057] In manufacturing the Nitinol stent, the material is first in theform of a tube. Nitinol tubing is commercially available from a numberof suppliers including Nitinol Devices and Components, Fremont Calif.The tubular member is then loaded into a machine which will cut thepredetermined pattern of the stent into the tube, as discussed above andas shown in the figures. Machines for cutting patterns in tubulardevices to make stents or the like are well known to those of ordinaryskill in the art and are commercially available. Such machines typicallyhold the metal tube between the open ends while a cutting laser,preferably under microprocessor control, cuts the pattern. The patterndimensions and styles, laser positioning requirements, and otherinformation are programmed into a microprocessor, which controls allaspects of the process. After the stent pattern is cut, the stent istreated and polished using any number of methods or combination ofmethods well known to those skilled in the art. Lastly, the stent isthen cooled until it is completely martensitic, crimped down to itsun-expanded diameter and then loaded into the sheath of the deliveryapparatus.

[0058]FIG. 15 illustrates an alternate exemplary embodiment of aself-expanding stent 1500 formed from Nitinol. In this exemplaryembodiment, a plurality of split-bridges are utilized to fill the gapbetween unbridged loops without serving as a structural connection pointbetween these loops. In stent designs featuring periodically placedbridges, as is described herein, the resulting gaps between unconnectedloops may be disadvantageous, especially during loading of the stentinto a stent delivery system and subsequent deployment from a stentdelivery system. In both the loading and deployment situations, thestent is constrained to a small diameter and subjected to highcompressive axial forces. These forces are transmitted axially throughthe stent by the connecting bridges and may cause undesirable bucklingor compression of the adjacent hoops in the areas where the loops arenot connected by bridges. A split-bridge may be utilized tosubstantially minimize this undesirable deformation under conditions ofconstrained axial compression. Essentially, when a stent, havingsplit-bridges is constrained and subjected to axial compression, thewide flat surfaces of adjacent ends of the split-bridge, as is explainedin detail subsequently, quickly come into contact and transmitcompressive axial loads without allowing undesirable deformation of thestent structure. The split-bridge design is particularly advantageous inthat it allows the transmission of the compressive axial loads duringstent loading and deployment without the loss of flexibility caused bystandard bridges once the stent is deployed.

[0059] A simple example may be utilized to illustrate the usefulness ofa stent comprising split-bridges. A constrained stent which comprisesthree bridges, typically spaced one hundred twenty degrees apart, musttransmit the entire compressive load associated with stent loading anddeployment through these three bridges. Unconnected loops within the onehundred twenty degree arc or span between bridges may be undesirablydeformed, potentially out of plane, thereby allowing compression of theentire stent structure and potentially adversely impacting loading ordeployment characteristics. However, a stent with three standard bridgesand three split-bridges would better distribute the axial compressiveload, with half the load at or on each bridge, now spaced apart by sixtydegrees. In this scenario, there are fewer unconnected loops within thesixty-degree arc or span and these loops would be less likely to becomeundesirably deformed when the structure is subject to compressive axialloads. Essentially, by allowing efficient transmission of compressiveaxial loads, the split-bridge helps to prevent undesirable compressionor deformation of the constrained stent and loading or deploymentdifficulties, which may result from such compression or deformation.This may facilitate loading and delivery of stent designs which mightotherwise be impractical.

[0060] It is important to note that symmetric loading and hencesymmetric placement of the bridges is preferable but not necessary.

[0061] Although the split-bridge design may be utilized in any number ofstent designs, for ease of explanation, the split-bridge design isdescribed with respect to the exemplary stent illustrated in FIG. 15. Asillustrated, the stent 1500 comprises a plurality of adjacent hoops1502. The hoops 1502 include a plurality of longitudinal struts 1504 anda plurality of loops 1506 connecting adjacent struts 1504, whereinadjacent struts 1504 are connected at opposite ends so as to form asubstantially S or Z shape pattern. The loops 1506 are curved,substantially semi-circular with symmetrical sections about theircenters 1508. The stent 1500 further comprises a plurality of bridges1510 which connect adjacent hoops 1502. The bridges 1510 are equivalentto the bridges illustrated in FIG. 5 and described above. Also asdescribed above, the bridge orientation is changed from hoop to hoop soas to minimize rotational changes between any two points on a givenstent during stent deployment or constraint. The number of and nature ofthe design of the struts, loops and bridges are important factors whendetermining the working properties and fatigue life properties of thestent as is discussed above.

[0062] The stent 1500 also comprises a plurality of split-bridges 1512.The split-bridges 1512 may comprise any suitable configuration and maybe positioned in any suitable pattern between bridges 1510. In theexemplary embodiment illustrated in FIG. 15, the split-bridges 1512 areorientated in a direction opposite from that of the bridges 1510 suchthat a symmetric configuration of bridges 1510 and split-bridges 1512results. As stated above, a symmetric configuration is not required, butit is preferred. Unlike the bridge 1510 design, the split-bridges 1512is designed to maximize the surface area for contact between adjacentsections of the split-bridges 1512. This split-bridge design allows forcontact between adjacent sections and thus force transmission even ifthe adjacent hoops 1502 become somewhat misaligned. The width orthickness of the split-bridges 1512 is preferably larger than the widthor thickness of the standard bridges 1510 to provide additional surfacearea for abutting contact. Similarly to bridges 1510, the split-bridges1512 have one end of one independent section attached to the one loop1506 and another end of one independent section attached to a loop 1506on an adjacent hoop 1502. Essentially, each split-bridge 1512 comprisesfirst and second independent sections which come into contact when thestent 1500 is under compressive axial loading and make no contact whenthe stent 1500 is deployed.

[0063] The geometry of the split-bridge may take any number of formswhich serves the purpose of filling the gaps which may be unoccupied bystandard bridges. In addition, the number and arrangement ofsplit-bridges is virtually unlimited.

[0064] By its nature, the split-bridge advantageously allowstransmission of compressive loads when constrained because the sectionsof each split-bridge abut at least partially. However, unlike atraditional bridge, it does not transmit tensile or compressive strainswhen the expanded structure is stretched, compressed or bent. Asillustrated in FIG. 16, the split-bridges are not aligned once thestructure is expanded. As such, the split-bridge may prove advantageousover a traditional bridge in contourability and fatigue durability.

[0065] Various drugs, agents or compounds may be locally delivered viamedical devices such as stents. For example, rapamycin and/or heparinmay be delivered by a stent to reduce restenosis, inflammation andcoagulation. One potential limiting factor in these stents is thesurface area available on the stent for the drugs, agents and/orcompounds. Accordingly, in addition to the advantages discussed above,the split-bridge offers additional surface area onto which variousdrugs, agents and/or compounds may be affixed.

[0066] As stated in previous sections of this application, markershaving a radiopacity greater than that of the superelastic alloys may beutilized to facilitate more precise placement of the stent within thevasculature. In addition, markers may be utilized to determine when andif a stent is fully deployed. For example, by determining the spacingbetween the markers, one can determine if the deployed stent hasachieved its maximum diameter and adjusted accordingly utilizing atacking process. FIG. 7 illustrates an exemplary embodiment of the stent100 illustrated in FIGS. 1-5 having at least one marker on each endthereof. In a preferred embodiment, a stent having thirty-six struts perhoop can accommodate six markers 800. Each marker 800 comprises a markerhousing 802 and a marker insert 804. The marker insert 804 may be formedfrom any suitable biocompatible material having a high radiopacity underX-ray fluoroscopy. In other words, the marker inserts 804 shouldpreferably have a radiopacity higher than that of the materialcomprising the stent 100. The addition of the marker housings 802 to thestent necessitates that the lengths of the struts in the last two hoopsat each end of the stent 100 be longer than the strut lengths in thebody of the stent to increase the fatigue life at the stent ends. Themarker housings 802 are preferably cut from the same tube as the stentas briefly described above. Accordingly, the housings 802 are integralto the stent 100. Having the housings 802 integral to the stent 100serves to ensure that the markers 800 do not interfere with theoperation of the stent

[0067]FIG. 8 is a cross-sectional view of a marker housing 802. Thehousing 802 may be elliptical when observed from the outer surface asillustrated in FIG. 7. As a result of the laser cutting process, thehole 806 in the marker housing 802 is conical in the radial directionwith the outer surface 808 having a diameter larger than the diameter ofthe inner surface 810, as illustrated in FIG. 8. The conical tapering inthe marker housing 802 is beneficial in providing an interference fitbetween the marker insert 804 and the marker housing 802 to prevent themarker insert 804 from being dislodged once the stent 100 is deployed. Adetailed description of the process of locking the marker insert 804into the marker housing 802 is given below.

[0068] As set forth above, the marker inserts 804 may be made from anysuitable material having a radiopacity higher than the superelasticmaterial forming the stent or other medical device. For example, themarker insert 804 may be formed from niobium, tungsten, gold, platinumor tantalum. In the preferred embodiment, tantalum is utilized becauseof its closeness to nickel-titanium in the galvanic series and thuswould minimize galvanic corrosion. In addition, the surface area ratioof the tantalum marker inserts 804 to the nickel-titanium is optimizedto provide the largest possible tantalum marker insert, easy to see,while minimizing the galvanic corrosion potential. For example, it hasbeen determined that up to nine marker inserts 804 having a diameter of0.010 inches could be placed at the end of the stent 100; however, thesemarker inserts 804 would be less visible under X-ray fluoroscopy. On theother hand, three to four marker inserts 804 having a diameter of 0.025inches could be accommodated on the stent 100; however, galvaniccorrosion resistance would be compromised. Accordingly, in the preferredembodiment, six tantalum markers having a diameter of 0.020 inches areutilized on each end of the stent 100 for a total of twelve markers 800.

[0069] The tantalum markers 804 may be manufactured and loaded into thehousing utilizing a variety of known techniques. In the exemplaryembodiment, the tantalum markers 804 are punched out from an annealedribbon stock and are shaped to have the same curvature as the radius ofthe marker housing 802 as illustrated in FIG. 8. Once the tantalummarker insert 804 is loaded into the marker housing 802, a coiningprocess is used to properly seat the marker insert 804 below the surfaceof the housing 802. The coining punch is also shaped to maintain thesame radius of curvature as the marker housing 802. As illustrated inFIG. 8, the coining process deforms the marker housing 802 material tolock in the marker insert 804.

[0070] As stated above, the hole 806 in the marker housing 802 isconical in the radial direction with the outer surface 808 having adiameter larger than the diameter of the inner surface 810 asillustrated in FIG. 8. The inside and outside diameters vary dependingon the radius of the tube from which the stent is cut. The markerinserts 804, as stated above, are formed by punching a tantalum diskfrom annealed ribbon stock and shaping it to have the same radius ofcurvature as the marker housing 802. It is important to note that themarker inserts 804, prior to positioning in the marker housing 804, havestraight edges. In other words, they are not angled to match the hole806. The diameter of the marker insert 804 is between the inside andoutside diameter of the marker housing 802. Once the marker insert 804is loaded into the marker housing 802, a coining process is used toproperly seat the marker insert 804 below the surface of the markerhousing 802. In the preferred embodiment, the thickness of the markerinsert 804 is less than or equal to the thickness of the tubing and thusthe thickness or height of the hole 806. Accordingly, by applying theproper pressure during the coining process and using a coining tool thatis larger than the marker insert 804, the marker insert 804 may beseated in the marker housing 802 in such a way that it is locked intoposition by a radially oriented protrusion 812. Essentially, the appliedpressure, and size and shape of the housing tool forces the markerinsert 804 to form the protrusion 812 in the marker housing 802. Thecoining tool is also shaped to maintain the same radius of curvature asthe marker housing 802. As illustrated in FIG. 8, the protrusion 812prevents the marker insert 804 from becoming dislodged from the markerhousing 802.

[0071] It is important to note that the marker inserts 804 arepositioned in and locked into the marker housing 802 when the stent 100is in its unexpanded state. This is due to the fact that it is desirablethat the tube's natural curvature be utilized. If the stent were in itsexpanded state, the coining process would change the curvature due tothe pressure or force exerted by the coining tool.

[0072] As illustrated in FIG. 9, the marker inserts 804 form asubstantially solid line that clearly defines the ends of the stent inthe stent delivery system when seen under fluoroscopic equipment. As thestent 100 is deployed from the stent delivery system, the markers 800move away from each other and flower open as the stent 100 expands asillustrated in FIG. 7. The change in the marker grouping provides thephysician or other health care provider with the ability to determinewhen the stent 100 has been fully deployed from the stent deliverysystem.

[0073] It is important to note that the markers 800 may be positioned atother locations on the stent 100.

[0074]FIG. 13 illustrates an alternate exemplary embodiment of aradiopaque marker 900. In this exemplary embodiment, the marker housing902 comprises flat sides 914 and 916. The flat sides 914 and 916 serve anumber of functions. Firstly, the flat sides 914 and 916 minimize theoverall profile of the stent 100 without reducing the radiopacity of thestent 100 under x-ray fluoroscopy. Essentially, the flat sides 914 and916 allow the marker housings 902 to fit more closely together when thestent 100 is crimped for delivery. Accordingly, the flat sides 914 and916 of the marker housing 902 allow for larger stents to utilize highradiopacity markers while also allowing the stent to fit into smallerdelivery systems. For example, the flat sides 914 and 916 on radiopaquemarkers 900 of the size described above (i.e. having appropriately sizedmarkers) allow a stent to maintain a radiopaque stent marker sizeutilized in a seven French delivery system to fit into a six Frenchdelivery system. Secondly, the flat sides 914 and 916 also maximize thenitinol tab to radiopaque marker material ratio, thereby furtherreducing the effects of any galvanic corrosion as described above. Themarker insert 904 and the marker hole 906 are formed of the samematerials and have the same shape as described above with respect toFIGS. 1-12. The markers 900 are also constructed utilizing the samecoining process as described above.

[0075]FIG. 14 illustrates yet another alternate exemplary embodiment ofa radiopaque marker 1000. This exemplary embodiment offers the sameadvantages as the above-described embodiment; namely, reduced profilewithout reduction in radiopacity and a reduction in the effects ofgalvanic corrosion. In this exemplary embodiment, the radiopaque marker1000 has substantially the same total area as that of the markers 900,800 described above, but with an oval shape rather than a circular shapeor circular shape with flat sides. As illustrated, the marker 1000comprises a substantially oval shaped marker housing 1002 and asubstantially oval shaped marker insert 1004. Essentially, in thisexemplary embodiment, the marker 1000 is longer in the axial directionand shorter in the radial direction to allow a larger stent to fit intoa smaller delivery system as described above. Also as in theabove-described exemplary embodiment, the nitinol tab to radiopaquemarker material ratio is improved. In addition, the substantially ovalshape provides for a more constant marker housing 1002 thickness aroundthe marker insert 1004. Once again, the markers 1000 are constructedfrom the same materials and are constructed utilizing the same coiningprocess as described above.

[0076] Any of the markers described herein may be utilized or any of thestent designs illustrated as well as any other stent requiring improvedradiopacity.

[0077] It is believed that many of the advantages of the presentinvention can be better understood through a brief description of adelivery apparatus for the stent, as shown in FIGS. 10 and 11. FIGS. 10and 11 show a self-expanding stent delivery apparatus 10 for a stentmade in accordance with the present invention. Apparatus 10 comprisesinner and outer coaxial tubes. The inner tube is called the shaft 12 andthe outer tube is called the sheath 14. Shaft 12 has proximal and distalends. The proximal end of the shaft 12 terminates at a luer lock hub 16.Preferably, shaft 12 has a proximal portion 18 which is made from arelatively stiff material such as stainless steel, Nitinol, or any othersuitable material, and a distal portion 20 which may be made from apolyethylene, polyimide, Pellethane, Pebax, Vestamid, Cristamid,Grillamid or any other suitable material known to those of ordinaryskill in the art. The two portions are joined together by any number ofmeans known to those of ordinary skill in the art. The stainless steelproximal end gives the shaft the necessary rigidity or stiffness itneeds to effectively push out the stent, while the polymeric distalportion provides the necessary flexibility to navigate tortuous vessels.

[0078] The distal portion 20 of the shaft 12 has a distal tip 22attached thereto. The distal tip 22 has a proximal end 24 whose diameteris substantially the same as the outer diameter of the sheath 14. Thedistal tip 22 tapers to a smaller diameter from its proximal end to itsdistal end, wherein the distal end 26 of the distal tip 22 has adiameter smaller than the inner diameter of the sheath 14. Also attachedto the distal portion 20 of shaft 12 is a stop 28 which is proximal tothe distal tip 22. Stop 28 may be made from any number of materialsknown in the art, including stainless steel, and is even more preferablymade from a highly radiopaque material such as platinum, gold ortantalum. The diameter of stop 28 is substantially the same as the innerdiameter of sheath 14, and would actually make frictional contact withthe inner surface of the sheath. Stop 28 helps to push the stent out ofthe sheath during deployment, and helps keep the stent from migratingproximally into the sheath 14.

[0079] A stent bed 30 is defined as being that portion of the shaftbetween the distal tip 22 and the stop 28. The stent bed 30 and thestent 100 are coaxial so that the distal portion 20 of shaft 12comprising the stent bed 30 is located within the lumen of the stent100. However, the stent bed 30 does not make any contact with stent 100itself. Lastly, shaft 12 has a guidewire lumen 32 extending along itslength from its proximal end and exiting through its distal tip 22. Thisallows the shaft 12 to receive a guidewire much in the same way that anordinary balloon angioplasty catheter receives a guidewire. Suchguidewires are well known in art and help guide catheters and othermedical devices through the vasculature of the body.

[0080] Sheath 14 is preferably a polymeric catheter and has a proximalend terminating at a sheath hub 40. Sheath 14 also has a distal endwhich terminates at the proximal end 24 of distal tip 22 of the shaft12, when the stent is in its fully un-deployed position as shown in thefigures. The distal end of sheath 14 includes a radiopaque marker band34 disposed along its outer surface. As will be explained below, thestent is fully deployed from the delivery apparatus when the marker band34 is lined up with radiopaque stop 28, thus indicating to the physicianthat it is now safe to remove the apparatus from the body. Sheath 14preferably comprises an outer polymeric layer and an inner polymericlayer. Positioned between outer and inner layers is a braidedreinforcing layer. Braided reinforcing layer is preferably made fromstainless steel. The use of braided reinforcing layers in other types ofmedical devices can be found in U.S. Pat. No. 3,585,707 issued toStevens on Jun. 22, 1971, U.S. Pat. No. 5,045,072 issued to Castillo etal. on Sep. 3, 1991, and U.S. Pat. No. 5,254,107 issued to Soltesz onOct. 19, 1993.

[0081]FIGS. 10 and 11 illustrate the stent 100 as being in its fullyundeployed position. This is the position the stent is in when theapparatus 10 is inserted into the vasculature and its distal end isnavigated to a target site. Stent 100 is disposed around stent bed 30and at the distal end of sheath 14. The distal tip 22 of the shaft 12 isdistal to the distal end of the sheath 14, and the proximal end of theshaft 12 is proximal to the proximal end of the sheath 14. The stent 100is in a compressed state and makes frictional contact with the innersurface 36 of the sheath 14.

[0082] When being inserted into a patient, sheath 14 and shaft 12 arelocked together at their proximal ends by a Tuohy Borst valve 38. Thisprevents any sliding movement between the shaft and sheath which couldresult in a premature deployment or partial deployment of the stent 100.When the stent 100 reaches its target site and is ready for deployment,the Tuohy Borst valve 38 is opened so that that the sheath 14 and shaft12 are no longer locked together.

[0083] The method under which the apparatus 10 deploys the stent 100 isreadily apparent. The apparatus 10 is first inserted into the vesseluntil the radiopaque stent markers 800 (leading 102 and trailing 104ends, see FIG. 7) are proximal and distal to the target lesion. Oncethis has occurred the physician would open the Tuohy Borst valve 38. Thephysician would then grasp hub 16 of shaft 12 so as to hold it in place.Thereafter, the physician would grasp the proximal end of the sheath 14and slide it proximal, relative to the shaft 12. Stop 28 prevents thestent 100 from sliding back with the sheath 14, so that as the sheath 14is moved back, the stent 100 is pushed out of the distal end of thesheath 14. As stent 100 is being deployed the radiopaque stent markers800 move apart once they come out of the distal end of sheath 14. Stentdeployment is complete when the marker 34 on the outer sheath 14 passesthe stop 28 on the inner shaft 12. The apparatus 10 can now be withdrawnthrough the stent 100 and removed from the patient.

[0084]FIG. 12 illustrates the stent 100 in a partially deployed state.As illustrated, as the stent 100 expands from the delivery device 10,the markers 800 move apart from one another and expand in a flower likemanner.

[0085]FIG. 17 illustrates yet another alternate exemplary embodiment ofa self-expanding stent 1700 formed from Nitinol. In this exemplaryembodiment, a plurality of anvil bridges are utilized to fill the gapbetween unbridged loops without serving as a structural connection pointbetween these loops. The split-bridge embodiment described abovecomprises a pair of structural elements originating from adjacent loopson adjacent hoops. These pairs of structural elements come into contactwhen the stent is placed in axial compression, thereby providing a meansfor transmitting axial loading without undesirable deformation of thestent structure or closing of the gaps between hoops and resultingcompression of the constrained structure. The anvil bridge embodiment;however, comprises only a single element which originates from one loopand comes into mating or abutting contact with a flattened area of atleast one adjacent loop on an adjacent hoop when under axialcompression.

[0086] As illustrated, the anvil bridge stent 1700 comprises a pluralityof adjacent hoops 1702. The hoops 1702 include a plurality oflongitudinal struts 1704 and a plurality of loops 1706 connectingadjacent struts 1704, wherein adjacent struts 1704 are connected atopposite ends so as to form a substantially S or Z shape pattern. Theloops 1706 are curved, substantially semi-circular with symmetricalsections about their centers 1708. The stent 1700 further comprises aplurality of bridges 1710 which connect adjacent hoops 1702. The bridges1710 are equivalent to the bridges illustrated in FIGS. 5 and 15 anddescribed above; however, the bridges 1710 may assume a variety ofalternate configurations. Also as described above, the bridgeorientation is changed from hoop to hoop so as to minimize rotationalchanges between any two points on a given stent during constraint ordeployment. The number of and nature of the design of the struts 1704,loops 1706 and bridges 1710 are important factors when determining theworking properties and fatigue life properties of the stent as isdiscussed in detail above.

[0087] The stent 1700 also comprises a plurality of anvil bridges 1712.The anvil bridges 1712 may comprise any suitable configuration and maybe positioned in any suitable pattern between bridges 1710. As set forthabove, symmetric loading and hence symmetric placement of the bridges,both regular and anvil type, is preferable but not necessary. In theexemplary embodiment illustrated in FIG. 17, the anvil bridges 1712 arecentered between bridges 1710 such that a symmetric configuration ofbridges 1710 and anvil bridges 1712 results. Unlike the split-bridgesdescribed above, the anvil bridge 1712 comprises a one-piece structurewhich extends from one loop 1706 on one hoop 1702 to a flat surface 1714formed in two loops 1706 on an adjacent hoop 1702 when the stent isunder axial compressive loading. The flat surface 1714 is preferable,but not necessary. The anvil bridge one-piece structure 1712 extendsfrom a single loop 1706 and has an increasing profile until it forms aflat anvil-like surface. The anvil design provides more contact surfacearea then other designs. This allows the design to be more tolerant ofmisalignments resulting from variation in constraint diameter andloading deformation. Since the anvil design results in a larger contactsurface area, the contact surface area on the adjacent hoop 1702requires the flat surface 1714 be formed from two longitudinallyadjacent loops 1706. Once again, it is important to note that the flatsurface 1714 is preferable, but not required. In addition, the contactsurface area may comprise one loop, two loops as described, or more than2 loops. Essentially, the anvil bridge itself is designed to maximizecontact surface area and the mating surfaces of the adjacent tips areflattened for the same reason. Furthermore, while the anvil bridge 1712originates from one loop 1706, the mating contact surfaces 1714 includetwo loops 1706. All of these factors allow for loading forces to bedistributed over a larger area. This allows for more uniformdistribution of loading forces throughout the structure, improvingstability and decreasing the likelihood of undesirable deformation whenthe constrained structure is loaded in axial compression.

[0088] The geometry of the anvil bridge may take any number of formswhich serves the purpose of filling the gaps which may be unoccupied bystandard bridges. In addition, the number and arrangement of axialbridges is virtually unlimited.

[0089] In this exemplary embodiment, the arrangement of the markers 800,standard bridges 1710 and anvil bridges 1712 was carefully chosen topromote purely axial load transmission through the stent 1700 structure.By its nature, the anvil bridge advantageously allows transmission ofcompressive loads when constrained because the anvil portion abuts ormakes at least partial contact with the contact surface 1714. However,unlike a traditional bridge, it does not transmit tensile or compressivestrains when the expanded structure is stretched, compressed or bent. Asillustrated in FIG. 18, the anvil bridges 1712 are not aligned once thestent 1700 is expanded. As such, the anvil bridge may provide advantagesover a traditional bridge in contourability and fatigue durability.

[0090] As stated above, various drugs, agents or compounds may belocally delivered via medical devices such as stents. For example,rapamycin and/or heparin may be delivered by a stent to reducerestenosis, inflammation and coagulation. The additional stent surfacearea provided by the anvil bridges would be beneficial if an additionalamount of the drug, agent, and/or compound is required for a specificdosing, release profile or the like.

[0091] Although shown and described is what is believed to be the mostpractical and preferred embodiments, it is apparent that departures fromspecific designs and methods described and shown will suggest themselvesto those skilled in the art and may be used without departing from thespirit and scope of the invention. The present invention is notrestricted to the particular constructions described and illustrated,but should be constructed to cohere with all modifications that may fallwithin the scope of the appended claims.

What is claimed is:
 1. An intraluminal medical device comprising: aplurality of hoops forming a substantially tubular member having frontand back open ends, the plurality of hoops comprising a plurality ofstruts and a plurality of loops connecting adjacent struts; one or morebridges interconnecting the plurality of hoops at predeterminedpositions to form the substantially tubular member; and at least oneanvil bridge positioned between each of the plurality of hoops, the atleast one anvil bridge including a first section which makes abuttingcontact with a section of at least one loop on an adjacent hoop when theintraluminal medical device is constrained and under compressive axialloading.
 2. The intraluminal medical device according to claim 1,wherein the plurality of hoops, the one or more bridges and the at leastone anvil bridge comprise a superelastic alloy.
 3. The intraluminalmedical device according to claim 2, wherein the superelastic alloycomprises from about fifty percent to about sixty percent Nickel and theremainder Titanium.
 4. The intraluminal medical device according toclaim 1, wherein the plurality of struts and the plurality of loops forma substantially S-shaped configuration.
 5. The intraluminal medicaldevice according to claim 1, wherein the one or more bridgesinterconnecting the plurality of hoops each comprise first and secondends, the first end being connected to a loop on one hoop and the secondend being connected to a loop on an adjacent hoop.
 6. The intraluminalmedical device according to claim 5, wherein the at least one anvilbridge positioned between each of the plurality of hoops are interposedbetween the one or more bridges interconnecting the plurality of hoops.7. The intraluminal medical device according to claim 6, wherein thefirst section comprises an enlarged section.
 8. The intraluminal medicaldevice according to claim 7, wherein the enlarged section of the atleast one anvil bridge comprises first and second ends.
 9. Theintraluminal medical device according to claim 8, wherein the first endof the enlarged section of the at least one anvil bridge is connected toa loop on one hoop and the second end of the enlarged section comprisesa flat surface configured to make at least partial abutting contact withthe flattened section of at least one loop on an adjacent hoop when theintraluminal medical device is constrained and under axial compressiveloading.
 10. The intraluminal medical device according to claim 9,wherein the second end of the enlarged section has an increased profilefor making abutting contact.
 11. The intraluminal medical deviceaccording to claim 9, wherein the enlarged section of the at least oneanvil bridge makes no contact with the flattened section when theintraluminal medical device is expanded.